My render is complete gray or transparent
First: Select the object you want to appear on the render. Check the eye icon is active. Check the camera icon is active.
Second: With the object selected, press "." on the numpad. This will focus on the selectd object. Now, press CTRL+ALT+"0" (zero on the numpad). This will make your current view your CAMERA view. This is ASSUMING you have a camera on scene. If not, create one, and go to step 1 again.
Finally, when you see your viewport and camera; on the outliner select your camera and go to CLIPPING. Set END to : 10 km. This should allow you to extensively look deep into your scene.
Finally if all of the above fails: Go to object properties (the cube icon on the properties region area) and scroll down to CYCLE settings: Make sure CAMERA is CHECKED for ray visibility:


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BFCT_Schiller